The invention relates to apparatus for use in positioning a unit opposite a tube of a regular array of tubes which open out onto a tube-plate and are housed in an enclosure provided with an access opening. It is particularly but not exclusively applicable to the positioning of a unit which is intended to be placed successively opposite all the tubes in a nest of boiler tubes of a steam generator of a nuclear power station, the unit being for example a guide-tube for a checking probe, a machining head or a welding head.
The very large number of tubes in the nest of boiler tubes of a steam generator of a pressurized water nuclear power station must be periodically checked in order to detect possible corrosion. This checking is usually carried out by means of an eddy current probe which is inserted successively into each of the tubes and is connected to apparatus for processing the signal from the probe. The probe is arranged inside a flexible guide-tube and brought successively opposite each of the boiler tubes which are to be checked. When the mouth of the guide-tube is correctly centered opposite one of the boiler tubes, the probe is propelled into the tube, for example by means of compressed air, and then withdrawn ready for insertion into another tube.
Attempts are obviously being made to reduce to a minimum the time required for checking all the tubes in the nest, in order to shorten the standstill time of the generator. Furthermore, the area of the boiler tubes is highly radioactive and it is absolutely obligatory to reduce human intervention to a minimum.
Hitherto, devices have been used in which the open end of the flexible guide-tube is carried by a carriage, which can be moved in two perpendicular directions, along guides which must be introduced through the manholes into the end water box of the nest and must be fixed to the plate. However, the steam generator, which must be pressure-resistant, is of cylindrical shape and the tube-plate is circular, with the result that, with these two perpendicular movements, the carriage cannot sweep the entire surface in order to enable the probe to reach all the tubes. The orientation of the fixed guides must consequently be altered several times.
This therefore makes it necessary for the operator to enter the water box, that is to say the zones of dangerous radioactivity, several times. The same also applies to the case where the tubes in the nest are U-shaped and the water chamber is divided into two compartments, with the result that the carriage can only sweep the surface of a semicircle.
Apparatus which limits human intervention is disclosed in U.K. Pat. No. 1,511,877 and U.S. Pat. No. 4,004,698 and comprises two arms which are each equipped with expansible mandrels capable of fixing themselves inside a tube. In use, each arm extends parallel to the plate and can move in the direction of its longitudinal axis relative to the other arm. The two arms can also be moved relative to one another, perpendicular to the plate. Thus, when the mandrels of one arm are fixed inside two tubes of the plate, the other arm can be moved along its axis and then towards the plate so as to engage its mandrels inside two tubes. The arm which was initially in position can then be disconnected from the tubes, moved away from the plate and fixed to two other tubes. This apparatus must initially be placed on the tube-plate by an operator who must therefore enter the steam generator.